Apparatus for treating diesel engine exhaust gas

ABSTRACT

An apparatus for treating Diesel engine exhaust gas includes a filter unit which is located in the upstream side of a flow passage in a Diesel engine exhaust duct, the filter unit being composed of a plural number of filter chambers which are thermally insulated from each other and selectively opened for admission of the exhaust gas. Located in the downstream side is a nitrogen oxide treatment section to induce reactions between nitrogen oxides and reductant gas components of the exhaust gas. The exhaust gas is admitted into one of the filter chambers in one time period to trap particulate material on a filter in a filter chamber while letting reductant gas components of the exhaust gas pass through toward the nitrogen oxide treatment section which is located in the downstream side. In the nitrogen oxide treatment section, reducing reactions are induced between nitrogen oxides and reductant gas components of the exhaust gas in the presence of a nitrogen oxide reduction catalyst. Concurrently, in a filter chamber which is disconnected from an exhaust gas inlet passage, a heater is turned on to burn off particulate matter trapped on a filter in a filter chamber.

TECHNICAL FIELD

This invention relates to an apparatus for treating Diesel engineexhaust, particularly suitable for use on a vehicle like a constructionmachine which is driven from a Diesel engine, for the purpose ofcleaning its exhaust gas by removal of toxic particulate substances andnitrogen oxides.

TECHNICAL BACKGROUND

As an internal combustion engine, Diesel engines excel gasoline enginesin heat efficiency and are at an advantage in fuel cost or mileage andother aspects except for the need for taking an ecologicalcountermeasure or countermeasures to cope with emissions of a largeamount of black smoke of so-called PM (particulate matter) in additionto nitrogen oxide (NOx). In this connection, there have thus far beenproposed various exhaust cleaning systems which are adapted to removeNOx and PM from exhaust gas. NOx are converted to N₂ by a reducingreaction before release to the atmosphere. On the other hand, PM isremoved by the use of a permeable filter which is capable of trapping PMwhile permitting passage of gaseous components of exhaust gas.

NOx are converted to N₂ gas by a reducing reaction in the presence of areducing agent or reductant. For example, disclosed in Patent Literature1 below is a NOx treatment system which is incorporated into an exhaustgas duct for treatment of nitrogen oxides. This treatment system isadapted to induce reducing reactions between nitrogen oxides NOx and areductant. In order to induce reducing reactions in an efficient andeffective manner, this treatment system is arranged to treat exhaustgases by reduction in the presence of a nitrogen oxide reductioncatalyst. In this regard, as a nitrogen oxide reduction catalyst, thesystem of Patent Literature employs a metal such as iridium or rhodiumin combination with a carrier of silica. Further, as a preferredreductant, a mention is made of hydrogen, carbon monoxide, hydrocarbons,and oxygen-containing compounds.

A system for removal of PM in exhaust gas is disclosed, for example, inPatent Literature 2 below. In Patent Literature 2, a filter forseparating PM components from other exhaust gas components is made ofceramic fiber having sufficient heat resistance along with air or gaspermeability. This filter is put in an exhaust gas flow to trapparticulate matter in the exhaust gas. The PM trapped on the filter,however, causes clogging of the filter, increasing pressure losses inthe exhaust gas flow and as a result making it difficult to secure asufficient flow rate of the exhaust gas. To cope with this problem, thepermeable filter is provided with a heater thereby to heat and burn offtrapped PM.

PRIOR ART LITERATURE(S)

-   Patent Literature 1: Japanese Patent Application Laid Open under    2004-73921-   Patent Literature 2 Japanese Patent Application Published under    4,023,514

SUMMARY OF THE INVENTION Problem(s) to be Solved by the Invention

In Patent Literature 1, as a preferred reducing agent or reductant foruse in detoxifying NOx in exhaust gas by conversion to NO₂, a mention ismade of hydrogen, carbon monoxide, hydrocarbons, oxygen-containingcompounds. On the other hand, in Patent Literature 2, trapped PM isburned off to prevent clogging of a filter, causing oxidation to carbonmonoxide and hydrocarbons in the exhaust gas which is passed through thefilter. That is to say, although a reductant is necessitated in the NOxtreating system of Patent Literature 1, reductant components of exhaustgas, including carbon monoxide and hydrocarbons, are consumed at thetime of burning off trapped PM in Patent Literature 2.

Therefore, in case the NOx treating technology of Patent Literature 1 iscombined with the PM stripping technology of Patent Literature 2, itbecomes difficult to utilize carbon monoxide and hydrocarbons whichexist in the exhaust gas as a reductant useful in the treatment of NOx.That is to say, in order to detoxify NOx and eliminate PM in Dieselengine exhaust gas on the basis of disclosures of Patent Literatures 1and 2, it becomes a requisite to provide an additional feed path for areductant which induces reducing reactions in the treatment of NOx. Inthis regard, it is conceivable to feed exhaust gas to a NOx treatingsystem in a stage anterior to removal of PM. However, in that case, onewill face a problem of contamination of a nitrogen oxide reductioncatalyst with PM to such a degree as to lose its functions as a catalystprematurely in an early stage.

Thus, it is difficult to construct an all-round cleaning system forDiesel engine exhaust gas, by simply combining the NOx treatingmechanism of Patent Literature 1 with the PM elimination mechanism ofPatent Literature 2.

In view of the foregoing situations, it is an object of the presentinvention to provide an apparatus for treating Diesel engine exhaust gasto effectively get rid of environmental pollutants such as PM and NOxwhich are contained in a large amount in Diesel engine exhaust gas.

Means for Solving Problem(S)

In order to achieve the above-stated objective, according to the presentinvention, there is provided an apparatus for treating Diesel engineexhaust gas, into which Diesel engine exhaust gas is introduced to getrid of particulate matter and to reduce nitrogen oxides in the exhaustgas to harmless nitrogen gas, characterized in that the apparatuscomprises: a filter unit located in an upstream side of a Diesel engineexhaust gas duct and composed of a plural number of filter chambers eachencasing a permeable filter member capable of trapping particulatematter in the exhaust gas while permitting passage therethrough ofreductant gas components in the exhaust gas, and a nitrogen oxidetreatment section located in a downstream side of the exhaust gas ductand encasing a nitrogen oxide reduction catalyst to induce reducingreactions between nitrogen oxides and the reductant gas components inthe exhaust gas; the filter chambers of the filter unit being thermallyinsulated from each other and each provided with a heater to burn offtrapped particulate matter; and a control means for the control ofexhaust gas feed to the respective filter chambers of the filter unit,the control means being adapted to feed exhaust gas successively to thefilter chambers, making an exhaust gas feed to one filter chamber in onetime period while suspending an exhaust gas feed to other filterchambers during that time period.

The filter in each one of the above-mentioned filter chambers is apermeable filter which is capable of trapping solid particulate matter(PM) like black smoke in the exhaust gas while permitting passagetherethrough of gaseous components of the exhaust gas. Diesel engineexhaust gas generally contains reductant gas components such as carbonmonoxide and hydrocarbons. These reductant gas components are allowed toflow out of a filter chamber without being consumed there, and utilizedas reducing agents in the nitrogen oxide treatment section which islocated downstream of the filter chamber. More particularly, a permeablefilter of ceramic fiber filter of good heat resistance is employed ineach one of the filter chambers in conditions which will preventoxidation reactions of the exhaust gas when it is admitted selectivelyinto one of the filter chambers and brought into contact with thefilter, letting reductant gas components in the exhaust gas passthrough.

As the filter is used continuously and repeatedly for trapping PM, it isinevitably clogged with trapped PM. The filter unit is provided with aplural number of filter chambers, so that, while one filter chamber isin a phase or stage of trapping PM, filters in other filter chambers areregenerated by getting rid of trapped PM. For this purpose, therespective filter chambers are successively connected to an exhaustinlet passage one after another by means of a shutter, which is operatedto connect the exhaust inlet passage selectively to one of the filterchambers in one time period while disconnecting other filter chambersfrom the exhaust inlet passage during that time period. A heater whichis provided in each one of the filter chambers is turned on to burn offtrapped PM when not in a PM trapping phase, thereby to restore thefilter performance. In this regard, it is preferred that each one of thefilter chambers be enshrouded in a heat insulating wall to thermallyinsulate a filter chamber in a PM trapping phase from a filter chamberor chambers which are in a heated state for restoration of filterperformance.

In addition to combustion of PM components, the heater which is providedin each filter chamber can be utilized for exhaust gas temperaturecontrol. That is, it is desirable to control the exhaust gas temperaturein such a range as would accelerate nitrogen oxide reducing reactions atthe time when NOx in the exhaust gas are reacted with a reductant in thepresence of a nitrogen oxide reduction catalyst in the nitrogen oxidetreatment section which is located downstream of the filter unit. In thecase of an iridium base catalyst, it is activated to a maximum degree ina temperature range of from 240° C. to 300° C. Accordingly, in a filterchamber which is in the PM stripping stage, it is desirable to controlthe heater in that chamber in such a way to heat the exhaust gas up to atemperature range which will contribute to acceleration of reducingreactions in the succeeding nitrogen oxide treatment section.

After removal of PM in a filter chamber, the exhaust gas which containsreductant gas components is introduced into the nitrogen oxide treatmentsection which is located downstream of the filter unit and packed with anitrogen oxide reduction catalyst. In this instance, various metaloxides can be used as a catalyst. More specifically, iridium or rhodiumcan be suitably used in combination with a carrier of porous silica. Inthe presence of a catalyst of this sort, nitrogen oxides (NOx) in theexhaust gas are converted to N₂ gas and H₂O by reactions with reductantgas components in the exhaust gas including carbon monoxide andhydrocarbons. Thus, the Diesel engine exhaust gas is cleaned by removalor reductions in amount of PM and NOx components.

PM which is contained in a large amount in Diesel engine exhaust gas issecurely trapped on a filter in one of the filter chambers underconditions which will not impair reductant gas components, and theexhaust gas which still contains NOx is introduced into the nitrogenoxide treatment section along with the reductant gas components to getrid of environmental pollutants in Diesel engine exhaust gas. Besides,while PM is removed in one filter chamber, filters in other chambers areregenerated by complete combustion of trapped PM.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of an apparatus for treating Diesel engineexhaust gas, according to the present invention.

FIG. 2 is a diagrammatic illustration explanatory of structural makeupof a filter chamber.

DESCRIPTION OF PREFERRED EMBODIMENT(S)

Hereafter, the present invention is described by way of its preferredembodiments. Shown in FIG. 1 is a system arrangement adopted by thepresent invention. In that figure, indicated at 1 is an exhaust duct forconduction of exhaust gas from a Diesel engine. A filter unit 2 isfitted in this exhaust duct 1 at an upstream end of the latter in thedirection of exhaust gas flow, and a nitrogen oxide treatment section 3is located in a downstream side of the exhaust duct 1.

The filter unit 2 is composed of a plural number of filter chambers 10each having a tubular outer casing 11 and a tubular inner casing 12which is located internally of the outer casing tube 11 as shown in FIG.2. In the upstream side of the exhaust duct 1, each filter chamber isprovided with a gas inlet plate 13 with an inlet opening 14 at aposition radially inward of the inner casing 12. The tubular innercasing 12 is formed of a porous plate to permit permeative circulationof an influent gas. On the other hand, the outer casing 11 is made up ofheat insulating walls. Further, an end closure plate 15 is fitted in theinner casing tube 12 at the downstream end of the filter unit 2. Anannular outlet opening 16 is formed between the outer and inner casings11 and 12.

Engine exhaust gas is admitted into one of the filter chambers 10through the inlet opening 14 and allowed to flow into the annular spacebetween the outer and inner casings 11 and 12, through the pores in theporous walls of the inner casing tube 12. Exhaust gas is brought intocontact with a filter 17 which is packed in the annular space betweenthe outer and inner casings 11 and 12 to trap particulate matter (PM)thereon. As a consequence, PM is filtered out and separated from theexhaust gas before it flows out through the outlet opening 16.

Trapped PM on the filter 17, including black smoke, is rendered harmlessby complete combustion. At the same time, the filter 17 can beregenerated by complete combustion of trapped PM on the filter 17. Forthis purpose, a heater 18 is provided internally of the inner casing 12of each filter chamber 10. The heater 18 is fixedly supported on the endclosure plate 15. Trapped PM in the filter 17 can be burned offcompletely by turning on the heater 18. Since the filter chamber 10 isheated to a high temperature at this time, the filter 17 is made ofceramic fiber with sufficient heat resistance.

In the case of the particular embodiment shown in FIG. 1, the filterunit 2 is composed of four filter chambers 10. By means of a rotaryshutter 20, the inlet openings 14 to the respective filter chambers 10are selectively connected to an exhaust inflow passage 1 a which isconnected to the exhaust duct 1. The rotary shutter 20 is provided witha communication port 21, by way of which the respective filter chambers10 are successively connected to the inflow passage 1 a one afteranother by rotation of the rotary shutter 20. That is to say, during atime period when one of the filter chambers 10 is connected to theexhaust inflow passage 10, other filter chambers 10 are disconnectedfrom the exhaust inflow passage 1 a.

The rotary shutter 20 is rotationally driven from a motor 22. The motor22 as well as the heaters 18 are put in operation under control of acontrol circuit 23. In the case of a Diesel engine vehicle, for example,in the case of a construction machine like hydraulic power shovel typeexcavator, a battery is mounted on the vehicle. Such an onboard batterycan be utilized as a power source 24 for the motor 22 and heater 18.

The nitrogen oxide treatment section 3, which is located downstream ofthe filter unit 2 in the exhaust duct 1, is constituted by a reactionchamber 30 which is packed with a nitrogen oxide reduction catalyst 31.The nitrogen oxide reduction catalyst 31 is constituted, for example, bya honeycomb structure of an occlusion material with good permeability.NOx is occluded in the occlusion material along with the reductioncatalyst to induce a reducing reaction, and resulting N₂ gas and H₂O aredischarged through an outflow passage 1 b. As a nitrogen oxidereductant, for example, an iridium or rhodium metal on a porous silicacarrier with a small amount of alkaline metal content can be suitablyapplied.

Further, as a reductant to be reacted with NOx, hydrogen, carbonmonoxide, hydrocarbons and oxygen-containing compounds can be used forthis purpose. Diesel engine exhaust gas contains large amounts ofreducing gas components including carbon monoxide and hydrocarbons.Thus, the exhaust gas itself can be used as a reductant. For thispurpose, the exhaust gas is retained in a non-oxidation state reactionsuntil it is introduced into the nitrogen oxide treatment section 3.

As described hereinabove, PM is trapped on the filters 17 in therespective filter chambers 10 of the filter unit 2 which is locatedupstream of the nitrogen oxide treatment section 3 in the exhaust duct1. Trapped PM is burned off in the filter chambers 10 by the heaters 18to regenerate the respective filters. However, if the exhaust gas is fedto a filter chamber 10 which is in a phase of burning off trapped PM,the reducing gas components of the exhaust gas will lose functions as areductant.

Of a plural number of filter chambers 10 of the filter unit 2,regeneration of a filter 17 by complete combustion of PM is not carriedout in a filter chamber 10 which is in the phase of separating andremoving PM from the exhaust gas. That is to say, regeneration of afilter 17 is not carried out as long as its filter chamber 10 isconnected to form part of a flow passage of the exhaust gas through theexhaust duct 1. It is after the filter chamber 10 has been disconnectedfrom the exhaust gas flow passage that the filter chamber 10 is heatedto a temperature for complete combustion of PM which has deposited onits filter 17. Therefore, the respective filter chambers 10 of thefilter unit 2 are thermally insulated from each other.

By adoption of the arrangements as described above, it becomes possibleto separate and remove toxic particulate matter (PM) which occurs inDiesel engine emissions, while converting NOx into a harmless form.Diesel engine exhaust gas in the exhaust duct 1 flows from the inflowpassage 1 a toward the outflow passage 1 b, but a heater 18 of a filterchamber 10 is not turned on when the filter chamber 10 is connected tothe inflow passage 1 a for removal of PM from the influent exhaust gas.Accordingly, while PM in the exhaust gas is removed and trapped on afilter 17 within a filter chamber 10, there is no possibility ofoxidization of reductants like carbon monoxide and hydrocarbons whichare contained in the exhaust gas. That is to say, after separation ofPM, the exhaust gas flowing out of each filter chamber 10 contains NOxalong with non-oxidized reductant gas components.

In this manner, while PM is trapped in one of the filter chambers 10 byits filtering function, heaters 18 in other filter chambers 10 areturned on one after another to heat the respective filter chambers up toa temperature at which trapped PM is burned off and removed by completecombustion for regeneration of the filters 17. While PM is filtered outin one of the four filter chambers 10, there is no need for concurrentlyburning off PM in all of the remaining three filter chambers 10. That isto say, it is possible to turn on and off the heaters 18 in relationwith intermittent 90 degrees revolutions of the rotary shutter 20,turning off a heater 18 of a filter chamber 10 which will come to a PMfiltering position next time, cooling off that filter chamber 10 beforestarting a PM filtering operation. On-off of the respective heaters 17is controlled by a control circuit 23. In this regard, arrangements maybe made to revolve the rotary shutter 20 intermittently at predeterminedtime intervals, or upon detection of a pressure loss in a filter chamber10 in a PM filtering phase, revolving the rotary shutter 20 whenpressure in a filter chamber 10 exceeds a predetermined value.

After removal of PM, the exhaust gas which contains NOx along withreductant gas components is admitted into a reaction chamber 30 of thenitrogen oxide treatment section 3. In the reaction chamber 30, theexhaust gas is brought into contact with a nitrogen oxide reductioncatalyst 31. As a consequence, NOx are occluded in the nitrogen oxidereduction catalyst 31 along with reductants such as carbon monoxide andhydrocarbons, and converted to harmless N₂ gas by reducing reactions inthe presence of the reductants just mentioned. Thus, after removal of PMcomponents and following detoxification of NOx components, cleanedexhaust gas is allowed to flow into the outflow passage 1 b of theexhaust duct 1 to prevent or suppress environmental pollutions whichwould otherwise be caused by Diesel engine emissions.

In this instance, in inducing reducing reactions between NOx andreductants such as carbon monoxide and hydrocarbons which are occludedin the nitrogen oxide reduction catalyst 31, the reductive reactions inthe reaction chamber 30 can be accelerated by activating the nitrogenoxide reduction catalyst 31. For instance, an iridium type catalyst canbe activated to a maximum degree in a temperature range between 240° C.and 300° C. Accordingly, the control circuit 23 at the control of theheaters 18 may be arranged not only to turn on those heaters 18 infilter chambers 10 which are not in a PM stripping phase while turningoff a heater 18 in a filter chamber which is in a PM filtering phase,but to turn on a filter 18 in a filter chamber 10 in a PM filteringphase to heat the exhaust gas up to such a temperature as willcontribute to activation of the nitrogen oxide reduction catalyst 31upon introduction into the reaction chamber 31.

DESCRIPTION OF REFERENCE NUMERALS  1: Exhaust duct  1a: Inflow passage 1b: Outflow passage  2: Filter unit  3: Nitrogen oxide treatmentsection 10: Filter chamber 11: Tubular outer casing 12: Tubular innercasing 16: Outlet opening 17: Filter 18: Heater 20: Rotary shutter 21:Communication passage 30: Reaction chamber 31: Nitrogen oxide reductioncatalyst

1. An apparatus for treating Diesel engine exhaust gas, into whichDiesel engine exhaust gas is introduced to get rid of particulate matterand to reduce nitrogen oxides in said exhaust gas to harmless nitrogengas, characterized in that said apparatus comprises: a filter unitlocated in an upstream side of a Diesel engine exhaust gas duct andcomposed of a plural number of filter chambers each encasing a permeablefilter member capable of trapping particulate matter in said exhaust gaswhile permitting passage therethrough of reductant gas components insaid exhaust gas, and a nitrogen oxide treatment section located in adownstream side of said Diesel engine exhaust gas duct and encasing anitrogen oxide reduction catalyst to induce reducing reactions betweennitrogen oxides and said reductant gas components in said exhaust gas;said filter chambers of said filter unit being thermally insulated fromeach other and each provided with a heater to burn off trappedparticulate matter; and a control means for the control of exhaust gasfeed to the respective filter chambers of said filter unit, said controlmeans being adapted to feed exhaust gas successively to said filterchambers, making an exhaust gas feed to one filter chamber in one timeperiod while suspending an exhaust gas feed to other filter chambersduring that time period.
 2. An apparatus for treating Diesel engineexhaust gas as set forth in claim 1, wherein each one of said filterchambers of said filter unit is packed with a permeable filter ofceramic fiber capable of filtering out particulate matter in saidexhaust gas while permitting passage of reductant gas componentsincluding at least carbon monoxide and hydrocarbons.
 3. An apparatus fortreating Diesel engine exhaust gas as set forth in claim 2, wherein saidfilter is packed between tubular inner and outer casings, of which saidinner casing having an inlet opening at one end of a closed porous tube,said heater being located within said inner casing and extended forwardtoward said inlet opening from a closed bottom end, and said outercasing being formed of a heat insulating material.
 4. An apparatus fortreating Diesel engine exhaust gas as set forth in claim 1, wherein saidfilter unit further comprises a shutter for covering and uncoveringinlet openings of the respective filter chambers under control of saidcontrol means, in such a way as to uncover an inlet opening of one ofsaid filter chambers to let said exhaust gas in, and turning on a heaterin other filter chamber to burn off trapped particulate matter on saidfilter while said inlet opening is closed by said shutter.
 5. Anapparatus for treating Diesel engine exhaust gas as set forth in claim4, wherein said control means is adapted to turn on a heater of a filterchamber in a phase of trapping particulate material, and to heat saidexhaust gas up to a temperature range suitable for activation of saidnitrogen oxide reduction catalyst in said nitrogen oxide treatmentsection.
 6. An apparatus for treating Diesel engine exhaust gas as setforth in claim 1, wherein said filter chambers of said filter unit arearranged in a circular formation and said shutter is in the form of arotary shutter adapted to uncover an inlet opening of one of said filterchambers selectively in one time period and provided with acommunication passage for introduction of said exhaust gas through theuncovered inlet opening.
 7. An apparatus for treating Diesel engineexhaust gas as set forth in claim 1, wherein said nitrogen oxidereduction catalyst is constituted by an occlusion material of ahoneycomb structure.
 8. An apparatus for treating Diesel engine exhaustgas as set forth in claim 7, wherein said occlusion material employsiridium or rhodium as a nitrogen oxide reduction catalyst on a porouscarrier of silica.